The present invention relates to an information transmission system and method, transmitting apparatus, and receiving apparatus for delivering information over a transmission path, such as via a satellite. In addition, the invention relates to a data processing device, a data processing method, and a recording medium, and in particular, relates to data processing devices, data processing methods and recording media for easily restricting user terminals from obtaining broadcast data, such as data broadcast over a satellite circuit.
Conventional digital satellite broadcasting systems utilize conditional access (CA) in which only legitimate subscribers who have signed up or contracted for reception are allowed to receive the broadcast.
In conditional access, a private key is given in advance to subscribers who have signed a contract for reception. A transmitter encrypts the broadcast data, using the private key, and transmits the data via a satellite. Then, the subscribers decode the received encrypted signals using the private key, which permits only those subscribers who have contracted for reception to watch and listen to the broadcast.
In recent years, satellite data transmission systems may transmit as part of a digital satellite broadcasting system. Because the satellite circuit has a much faster transmission speed when compared to other systems, such as standard telephone circuits and ISDN, large amounts of data may be transmitted in a short time.
In the satellite data transmission system, various reception controls may be used for (i) general message communication to transmit the same data to all recipients (known as a “broadcast”), (ii) group communication to transmit the same data to a specific group of recipients (known as a multicast), or (iii) individual communication to transmit a different set of data to each individual (known as a “uni-cast). Thus, the potential uses for a satellite data transmission system are significantly increased.
The conditional access system, however, has the problem that reception control cannot be used for a uni-cast or multicast communication because this system is designed with the assumption that all the recipients receive and watch the same information.
Further, a greater plurality of channels may be delivered in the same band as digital data that is transmitted in the form of analog signals. Also, higher quality images and sounds are provided when transmitted in the form of digital data. Thus, satellite broadcasting and satellite communication systems, which provide images and sounds in the form of digital data, are proliferating. Such digital satellite broadcast services include SkyPerfect TV! and DirecTV in Japan, DirecTV in the United States, and Canal Plus in Europe. The digitalization of broadcasts reduces the broadcast cost per channel and provides programs and data that are processed by computer. Also, digitalization permits the widespread use of services in which programs, images, etc. are provided that are linked to each other.
In a digital satellite broadcast service, digital data representing images and sounds are converted into a format, such as the Moving Picture Experts Group (MPEG)2 format or the Digital Video Broadcasting (DVB) format which is derived from the MPEG 2, and then multiplexed for transmission in the form of radio waves. The radio waves are transmitted and received by the transponder of a satellite, where the radio waves are amplified and subjected to other processes for re-transmission to the earth.
The transmission band for the transponder may be as wide as 30 Mbps (Megabits per second) so that digital data of high quality may be distributed at high speed utilizing the whole width of the band. Though the actual transponder transmission band is 30 Mbps, a real transmission band is around 27 Mbps, at most, to allow the inclusion of error correction codes.
Generally, the transmission band for the transponder is divided into many bands of multiple channels to reduce cost. Though the content of the digital data transmitted on each channel differs, the mechanism by which the receivers receive the digital data on each channel remains the same. Consequently, a conditional access (CA) mechanism is needed to allow only permitted users to receive the digital data.
For data broadcast, in particular, the quantity of data per program is small when compared to the images or sounds distributed so that a charging unit or charging system is expected to become more complex. Therefore, a conditional access mechanism capable of performing more specific reception control is needed to address such a problem. The conditional access mechanism is also required to prevent passage of secret information during distribution.
Generally, conditional access is attained by performing encryption on a data stream before it is distributed. Two types of encryption methods are known, namely (i) a common key cryptosystem, also known as a private key cryptosystem, and (ii) a public key cryptosystem. For digital satellite broadcasting, the common key cryptosystem is more common because of a smaller number of encryption/decryption processes are used when compared to the public key cryptosystem.
In the common key cryptosystem, a row of codes that comprise a decryption key and correspond to an encryption key is given to a subscriber A by some method. Data is encrypted for distribution using the encryption key. The encrypted data is designed to make it hard to derive the encryption key, decryption key or the original data, whether by converse calculations or other means. Thus, a non-subscribed user B cannot accurately restore the original data even if the user B receives the encrypted data. On the other hand, the subscribed user A can restore the original data by decrypting the encrypted data using the decryption key given when the contract is made. Therefore, the making of a contract for reception is equivalent to reception of a decryption key.
When both users A and C are subscribers, for example, and the contract with A expires, or when the user A does a wrong action, the current encryption key is changed, and a decryption key equivalent to the new encryption key is provided to user C only. Thus, the user A who was previously a subscriber or did a wrong act cannot decode data encrypted with the new encryption key, whereas the legally subscribed user C can readily decode the data encrypted with the new encryption key.
It is difficult, however, to alter an encryption key, and it is further difficult to provide a new decryption key corresponding to a new encryption key to lawful subscribers whenever the subscription of another user expires or whenever improper conduct is discovered.